Field of the Invention
The present invention relates to reverse osmosis and/or ultrafiltration spiral wound membrane cartridges for liquid separation.
Discussion of the Related Art
Ultrafiltration membranes, also known as permeable membranes, thin film composite (TFC) membrane, or reverse osmosis (RO) membranes, and their use in spiral wound modules or modules are ubiquitous. The use of ultrafiltration membranes in liquid separation through reverse osmosis has particularly become important in water desalination. Ultrafiltration membranes sheets typically include a felt backing layer and a support layer and a discrimination layer. The discrimination layer can be a polyamide film that can be used to filter the liquid. The support layer is located between the discrimination layer and the felt backing layer and generally is a porous polymer material such as polysulfone to allow the flow of filtered fluid. The felt backing layer is typically porus so as to allow fluid that has been filtered by the discrimination layer to flow through the membrane sheet and into the collection line around which the ultrafiltration sheets are wound. The collection lines typically comprise an inner tube that has holes to intake the fluid filtered by the ultrafiltration membranes. Exemplary embodiments of filtration modules are disclosed in, for example, U.S. Patent Application Publication No. 2013/0199988, which is incorporated herein by reference.
In forming the spiral module, leafs of ultrafiltration membranes are typically sandwiched in an assembly including a length of an ultrafiltration membrane sheet folded in half with the fold adjacent an inner tube, and with a length of a spacer or feed carrier material positioned between the sheet halves. This method involving the folding of the membrane sheet, however, typically causes cracks in the discrimination layer of the membrane sheets along or near the crease of the fold line. Cracks in the discrimination layer are not desirable because they result in lower rejection rates by allowing unfiltered fluid to reach the inner tube.
Different approaches have been attempted to resolve this issue with the fold line. One common approach is to use tape, such as polypropylene adhesive film, on the fold line either on one or both sides of the membrane sheet. However, use of tape has some drawbacks.
On the active side of the membrane the tape has been known to come off during use or cleaning. This is especially so when an antifouling layer of water soluble polyvinyl acetate (PVA) is formed on the discrimination layer. As the PVA dissolves during use, it affects the adhesion of the tape to the active layer side of the membrane. As the tape peels off, the cracks in the discrimination layer become expose and thus allow flow of unfiltered fluid through the membrane sheet.
When the tape is placed on the felt backing layer, the pressure of the water flowing through the felt layer can cause the tape to come loose and consequently to also allow for unfiltered fluid to flow into the inner tube.
Moreover, the use of tape can greatly increase the thickness of the membrane sheet at the fold line. This large thickness variation between the fold line and the rest of the membrane sheet creates additional stress on the discrimination layer during module assembly and operation. The additional stress can lead to more cracks that again would result in decreased effectiveness of the discrimination layer filtration.
Another approach to addressing this issue is described in U.S. Pat. No. 4,842,736, which is incorporated herein by reference. In that process the fold line is reinforced by application of a flexible sealant. Specifically, a sealant is impregnated into the membrane sheet so as to fill the voids of the felt backing layer and support layer and preferably penetrate completely up to the discrimination layer. This results in an impermeable area where the membrane sheet is folded. The sealant used can be either an adhesive or a tape that is applied via heat and pressure and which remains flexible after application, thereby still allowing the impregnated membrane sheet to be folded. In this manner, even if cracks form in the discrimination layer, no unfiltered fluid will flow through them.
In yet another approach to resolve this problem, U.S. Patent Application Publication No. 2013/0199988, which is incorporated herein by reference, provides two additional methods to avoid the problems of forming cracks in the discrimination layer of the ultrafiltration membrane. In this disclosure the membrane sheet is not folded. Instead, two membrane sheets are secured together to form a leaf. In one embodiment, the two membrane sheets are secured using a tape. In another embodiment the use of tape or adhesive is optional as one end of two membrane sheets are laminated together using heat and pressure.
Although the above and other methods have been employed with mixed success in avoiding defects at the fold line, they present very complex and expensive processes. They also include the use of adhesives and tapes that are required to be properly positioned and secured or impregnated into the membrane sheets. These processes encumber the manufacturing process of the spiral modules and require the expense for the additional time and materials.
In view of the above discussed disadvantages, there is a need for an improved method to reinforce the fold line that is more efficient but still effective.